Its URL was given in a previous post in this thread. (It was posted on the Olympus forum, under a previous user name for the same person)

I am now enlightened.

As he clearly defines it, "Equivalent images are simply images which have the same perspective, framing, DOF, shutter speed and display dimensions." and "Fully equivalent images will be made from the same total amount of light. ... the noise will be the same for equivalent images, but not for the same exposure."

So, that is clear. Now, it is up to each of us to decide whether this is worth our time to think about. Clearly, I was thinking about 50mm lens on FF and 25mm lens on 4/3 to span the same angle, but that is not what he's aiming at. I was aiming at having equally bright images - I would say equal exposures, but then we'd have to argue about that - which would require the same f stop at the same shutter speed and ISO. The images would have different noise.

So, now that I understand that equivalent images on same efficiency sensors will have the same noise but not the same exposure, it's clear that I was not talking about the same thing at all.

I am very sure that whatever you call the concept I'm thinking of it is useful to me. I'll work on understanding how equivalence as defined in the FAQ is useful to me.